The present invention relates generally to gas turbine engines, and, more specifically, to air compressors therein.
A typical aircraft turbofan gas turbine engine includes a multistage axial compressor for sequentially pressuring air. The compressor includes a rotor having a plurality of axially spaced apart rows of compressor rotor blades extending radially outwardly therefrom. Surrounding the rotor is an annular casing from which extends radially inwardly a plurality of rows of compressor stator vanes which cooperate with respective blade rows for compressing the air in stages.
A fixed stator vane stage is typically formed in a plurality of circumferentially adjoining sectors which are removably attached to the casing. Each sector includes an arcuate outer band, an arcuate inner band, and several stator vanes extending radially therebetween. The outer band includes forward and aft rails which engage corresponding hooks or slots in the casing for mounting the sectors thereto. The inner band is suspended radially outwardly of the compressor rotor and axially between adjacent rows of rotor blades.
Since the blades sequentially pressurize the air from stage-to-stage, a differential pressure exists axially across each of the stator stages. Accordingly, an interstage seal is mounted from the inner bands and cooperates with a plurality of sealing teeth extending radially outwardly from the compressor rotor for effecting a labyrinth seal at each stator stage.
The interstage seal is typically attached to the compressor sectors by a backing strip having opposite axial rails which engage complementary hooks formed in the inner bands. A seal pad is attached to the backing strip and is typically in the form of a honeycomb for cooperating with the rotor teeth and effecting a fluid seal.
Since the compressor sections and interstage seals are fabricated assemblies, they are subject to typical manufacturing tolerances and assembly stackup. These components are typically manufactured from sheet metal which experiences variability in the assembly of the seal strips into the inner bands. The seal mounting hooks on the inner band are typically C-section sheet metal portions which are also arcuate in the circumferential direction along the sector. The corresponding rails of the backing strip must be similarly arcuate in curvature so that they may be assembled by circumferential insertion into the corresponding C-hooks.
In this arrangement, radial clearance is necessarily found between the rails and the mounting hooks to permit sliding assembly therebetween without excessive friction. This clearance, however, leads to vibratory wear during operation which can adversely affect the useful life. Furthermore, manufacturing differences in curvature of the rails and the mounting hooks effect point contacts therebetween which localize wear and decrease friction damping during operation.
In one design, the mounting hooks are crimped at several locations after assembly of the seal to the inner band for reducing the clearances therebetween and to increase friction damping. However, the sheet metal components have inherent resiliency which prevents the complete elimination of clearance therebetween even after the crimping operation. And, disassembly requires opening the crimps, which results in local damage.
Furthermore, since the seal is subject to occasional rubs by the rotor seal teeth during operation, suitable stops are provided in the inner band to prevent circumferential rotation of the seal segments therein. In one design, one of the circumferential ends of the C-hooks is crimped to effect such a stop. Rub reaction loads are therefore concentrated at these individual stops which increases the stress thereat.
The inherent looseness of the seal in the inner band, and vibratory and rub loads at local contact points cause associated wear thereat which can significantly reduce the useful life of the seal, or sector, or both. Accordingly, it is desired to provide an interstage seal having an improved mounting to the compressor stators for reducing wear and increasing damping thereof.